Method for forming a uniform continuous web of paper

ABSTRACT

A METHOD AND APPARATUS ARE DISCLOSED FOR FORMING A CONTINUOUS WEB OF PAPER UTILIZING THE PRINCIPLES OF ELECTROPHORESIS AND ELECTRO-OSMOSIS. A UNIFORM AQUEOUS DISPERSION OF PAPER-MAKING FIBERS IS FLOWED BETWEEN A PAIR OF HORIZONTAL, SPACED-APART, ELECTRICALLY CHARGED ENDLESS BELTS WHICH SERVE AS ELECTRODES FORMING A DEPOSITION ZONE. THE PAPER-MAKING FIBERS ARE ATTACHED TO ONE ELECTRODE BY ELECTROPHORESIS AND WATER TO THE OTHER ELECTRODE BY ELECTROOSMOSIS. A ELECTROMECHANICAL VIBRATOR VIBRATES THE TROUGH JUST PRIOR TO ENTRY OF THE AQUEOUS DISPERSION BETWEEN THE ELECTRODES TO MAINTAIN THE FIBERS IN SUSPENSION AND UNIFORMLY DISPERSE THEM WITHOUT FLOCCULATION. ELECTRICALLY NONCONDUCTING GUIDE BLOCKS ARE MOUNTED IN CONTACT WITH THE EDGES OF THE ENDLESS BELTS IN THE AREA OF   THE ELECTROSTATIC FIELD TO MAINTAIN THE PROPER ELECTRODE SPACING. AFTER THE ELECTRODE AND FIBER MAT ARE REMOVED FROM THE DEPOSITION ZONE, THE FIBER MAT IS SEPARATED FROM THE ELECTRODE SURFACE BY AN ADJACENT ELECTRICALLY CHARGED SURFACE WHICH TRANSFERS THE MAT. RESIDUAL WATER IN THE MAT IS REMOVED SIMULTANEOUSLY WITH THE TRANSFER BY ELECTROOSMOSIS. THE CELLULOSE FIBER MATS, PARTICULARLY USEFUL AS CAPACITOR TISSUE, HAVE BETTER ELECTRICAL PROPERTIES THAN CAN BE OBTAINED ON CONVENTIONAL PAPER-MAKING MACHINES.

L- W- STILES Dec. 12, 1972 METHOD FOR FORMING A UNIFORM CONTINUOUS WEBOF PAPER Filed Sept. 21. 1970 //Vl/ENT0R1 LESTEFM 577155 15 UnitedStates l atent O 3,705,847 METHOD FOR FOG A UNIFORM CONTINUOUS WEB OFPAPER Lester W. Stiles, Longview, Wash, assignor to WeyerhaeuserCompany, Tacoma, Wash. Filed Sept. 21, 1970, Ser. No. 73,779 Int. Cl.B011; /02

U.S. Cl. 204181 4 Claims ABSTRACT OF THE DISCLOSURE A method andapparatus are disclosed for forming a continuous web of paper utilizingthe principles of electro phoresis and electro-osmosis. A uniformaqueous dispersion of paper-making fibers is flowed between a pair ofhorizontal, spaced-apart, electrically charged endless belts which serveas electrodes forming a deposition zone. The paper-making fibers areattracted to one electrode by electrophoresis and water to the otherelectrode by electroosmosis. An electromechanical vibrator vibrates thetrough just prior to entry of the aqueous dispersion between theelectrodes to maintain the fibers in suspension and uniformly dispersethem without flocculation. Electrically nonconducting guide blocks aremounted in contact with the edges of the endless belts in the area ofthe electrostatic field to maintain the proper electrode spacing. Afterthe electrode and fiber mat are removed from the deposition zone, thefiber mat is separated from the electrode surface by an adjacentelectrically charged surface which transfers the mat. Residual water inthe mat is removed simultaneously with the transfer by electroosmosis.The cellulose fiber mats, particularly useful as capacitor tissue, havebetter electrical properties than can be obtained on conventionalpaper-making machines.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to a method and apparatus for forming continuous webs of paperthrough use of electrophoretic and electro-osmotic phenomenon from anaqueous dispersion of papermaking fibers.

(2) Prior art relating to the invention Conventional paper-makingmachines form a paper mat by retaining the paper-making fibers on a wiremesh, e.g., Fourdrinier wire. The water in the aqueous dispersion ofpaper-making fibers is forced through the wire by gravity, vacuum,surface tension, or mechanical pressing. Fine fibers and other particlesare removed with the water by these methods. In the apparatus and methodof this invention no Fourdrinier wire is used. In its place are a pairof spaced-apart, oppositely charged conducting surfaces serving aselectrodes.

Electrodeposition by electrophoresis is well known and is used forindustrial painting and for forming insulating coatings on metals. Forexample, U.S. Pat. Nos. 892,188 and 894,070 describe methods andapparatus for extracting water or other liquids from such materials aspeat pulp. Other known methods and apparatus are shown by the followingpatents to Huebner, U.S. Pat. Nos. 2,224,- 391; 2,615,822; and2,680,079.

It was not heretofore known, however, that a continuous web of papercould be formed by electrophoresis, nor that a machine to form acontinuous web of paper by electrophoresis could be devised. In theapparatus of the present invention only papermaking fibers are consumed.The Water is removed continuously from the aqueous dispersion of fibersand can be reused in processing more stock.

Patented Dec. 12, 1972 The method and apparatus of this invention areprimarily useful in making thin capacitor tissue. Thin capacitor tissuemade with the method and machine of this invention includes a higherpercentage of fibers, col loidal particles and other material, such fineparticles usually not retained when a Fourdrinier wire is used. Inaddition, a capacitor sheet can be made with .greater uniformity becausethe fibers and other particles in the stock are continuously beingfurnished to fill open pores and thin areas.

SUMMARY OF THE INVENTION A uniform aqueous dispersion of paper-makingfibers is continuously flowed at a regulated rate between a pair ofspaced-apart, continuously moving, oppositely charged endless beltswhich serve as conducting surfaces or electrodes and form a depositionzone. The fibers are generally attracted and migrate to the positiveelectrode or conducting surface and are deposited as a fiber mat on thesurface of the endless belt. The water is attracted by electro-osmosisand gravity to the opposite electrode. The fiber mat formed on thepositively charged conducting surface is removed from the conductingsurface and transferred to another surface by positively charging anadjacent moving surface in contact with the mat. Additional water in themat is also removed during the transfer by electro-osmosis.

A vibrating trough, mounted on the machine just prior to entrancebetween the two conducting surfaces, maintains the fibers in suspensionand uniformly dispersed without flocculation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a verticaltransverse section of one form of the apparatus for forming paper webs;and

FIG. 2 is a section view along line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1,the apparatus of this invention comprises stock chest 2 in whichpaper-making stock 1, consisting of pulp fibers and water, is stored.The stock at this point is made up to a predetermined consistency andbeating degree. An agitator 3 is run at a speed which prevents settlingbut does not cause introduction of air. Submerged pump 4 transfers thestock to head box 6 through line 5. Excess stock is removed to the stockchest by line 11. The slice 9 is set at a level so that the stock levelis just slightly above the upper electrode 13. The head adjustment 10 isset to provide a predetermined flow rate. An electromechanical vibrator7 is attached to a section of trough 8 to maintain the fibers insuspension and uniformly disperse them without flocculation. Vibrationof this section of the trough is isolated from the ends of the trough byrubber connection 8a and 8b. The vibrator used, for example, may be oneoperated on a sixty cycle line driving the blade at cycles per second.The aqueous dispersion of paper-making fibers is fed from head 'box 6through trough 8 between a set of endless, flexible conducting beltswhich serve as electrodes forming a deposition zone 50. The rate of flowof paper-making fibers between the belts can be adjusted by adjustmentof slice 9. The belts may be of stainless steel or other conductingmaterial.

The electrodes are oppositely charged. Usually the upper electrode ispositively charged and the lower electrode negatively charged. Sincepulp fibers are normally negatively charged they are attracted to andmigrate to the positive electrode and the water migrates to the lower ornegative electrode. The electrodes can be made as wide as desired forthe size of paper sheet wanted. The two conducting surfaces 12 and 13are in continuous movement. The speed at which the conducting surfacesare run for adequate paper formation can be theoretically calculatedbased on moving a charged particle from the surface of the negativeelectrode to the surface of the positive electrode according to theclassical Helmholtz equation:

where v is the average velocity of a particle E is the electrode voltagedifference,

L is the electrode separation,

Z is the Zeta potential of the particles, D is the dielectric constantof water, and 1 is the viscosity of water.

The speed is proportional to the active electrode length as can be seenfrom the equation. Endless belts 18a or 18b of rubber or similarmaterial form seals at the edges of the electrodes as can be best seenin FIG. 2. Other nonconducting materials than rubber can be used wheresuitable. Endless belts 18a and 18b are trained over idler pulleys 40aand 40b which guide the belt seals. Stationary nonconducting blocks 19a,19b, 20a and 20b are mounted adjacent the edges of electrodes 12 and 13.The electrodes slide over the blocks which function to maintain theproper electrode gap between electrodes 12 and 13. It is essential thatspacing of the electrodes be maintained constant in order to obtain auniform web of paper.

As the aqeuous dispersion of paper-making fibers is fed between theelectrodes 12 and 13 into deposition zone 50, the papermaking fibersmigrate to the upper or positive electrode by electrophoresis and thewater is attracted to the lower electrode by electro-osmosis andgravity. A fiber mat 23a is thus formed on the upper belt electrode 13.The water flows by gravity into tank 22. Doctor blade 26 and 27 can beplaced as shown in the drawing to scrape the respective conducting beltsclean and dry before deposition of additional fibers.

The fiber mat 23a clings to electrode surface 13 as it passes out of thearea of electrostatic force in the deposition zone 50 between electrodesurfaces 12 and 13. To remove the fiber mat from the surface ofelectrode 13 it is necessary to contact the fiber mat with a morepositively charged surface. As shown in FIG. 1 a papermaking press felt25, trained around electrically conducting rolls, driven and supportedby rolls 26 and 27, picks up the mat from upper electrode surface 13. Anidler roll 29 guides the paper-making felt tangent to drum 17 and theupper belt 13 to provide optimum pressing conditions. The spacingbetween roll 17 and 26 is adjusted for optimum pressing and pickup. Roll26, being positive with respect to the upper electrode 13, transfers thefiber mat 23a to the paper-making felt by electrophoresis. Residualwater in the mat is removed at the same time by electro-osmosis andflows to collection area 31 where it is pumped out by line 32. Roll 24functions to form area 31 so as to catch the residual water.

Roll is driven in the direction shown and is positively charged withrespect to roll 27 so that mat 23b is transferred to roll 30 byelectrophoresis. Residual water is forced into the felt 25 byelectro-osmosis and removed by a suction box or other suitable means 28.The fiber mat 23c is then drawn off roll 30 and is further pressed anddried by conventional means.

The electrode surfaces 12 and 13 and the pickup rolls 26, 27 and 30 ofFIG. 1 are powered by a direct current power source. A suitable powercircuit, for example, is an electrode power circuit capable ofdelivering about 500 volts negative potential with respect to a common 4ground and a pickup power circuit capable of delivering about voltspositive potential with respect to a common ground. Contacts 35 and 38of FIG. 1 are connected to both power supplies. Contacts 37 and 39 areattached to positive output terminals while contact 36 is connected to anegative output terminal.

The apparatus of this invention is primarily suitable for making thincapacitor tissue of increased uniformity over that made usingconventional paper-making machinery. Capacitor tissue produced by theapparatus of this invention is superior to capacitor tissue made onconventional paper-making machinery. Such paper has the intrinsiccharacteristic of greater inclusion of fines, colloidal and otherparticles which contribute to its uniformity, and consequently itselectrical insulating properties.

EXAMPLE A sheet of thin capacitor tissue was made by passing an aqueousdispersion of paper-making fibers at a temperature of approximately F.and a consistency of approximately 0.5% by weight of pulp between a pairof horizontally spaced-apart electrode surfaces such as shown in FIG. 1.The active electrode length was 12" and the width 3". The spacingbetween the electrodes was maintained at A2" and the machine speed atapproximately 10 feet per minute. An electrode potential difference of50 volts was maintained between the electrodes 12 and 13 at a currentflow of 2 amperes. Pickup rolls 26 and 30 were maintained at a potentialdilference of volts from that of conducting electrode surface on whichthe paper-making fibers were deposited. During passage between theelectrode surfaces the paper-making fibers migrated and were depositedin a uniform manner on the surface of the positively charged upperelectrode surface. The water migrated toward the lower or negativeelectrode and subsequently ran into a storage tank. The water removed byelectro-osmosis was examined and observed to be much cleaner than waterfrom conventional paper-making machines, indicating that more of thefibers were being deposited in the formation of the fiber mat and notbeing lost.

The paper made by the above apparatus was found to have superiorelectrical formation as determined by a Mercury Electrode Tester ascompared to conventional capacitor tissue made on standard paper-makingmachinery. Electrophoretic samples tested at 100 volts or more had 18%fewer counts per foot than standard capacitor tissue. Electrophoreticsamples formed at 1 /2 amperes and tested at 100 volts compared withstandard paper, had 55% fewer counts per foot. The above values showconclusively that the paper formed by the above electrophoretic methodhas superior electrical formation.

What is claimed is:

1. A method for continuously making paper webs from an aqueousdispersion of paper-making fibers which comprises:

(1) continuously flowing at a regulated flow rate a uniform aqueousdispersion of paper-making fibers between a spaced-apart set ofcontinuously moving, oppositely charged conducting surfaces, theconducting surfaces creating between them an electrostatic fieldextending the length and transverse width there- I (2) maintaining theelectrostatic field between the conducting surfaces at a predeterminedpotential such that the paper-making fibers in the dispersion migrate toand are deposited by electrophoresis on one of the conducting surfaces,forming a fiber mat of predetermined thickness,

(3) removing the fiber mat from the conducting surface and, at the sametime, removing residual water from the paper mat by bringing the matinto contact with 5 6 a charged surface having a potential sufficient totional forces sufficient to maintain the fibers in suspension attractthe paper mat to said charged surface, and and uniformly dispersedwithout flocculation.

(4) drying and pressing the mat.

2. Method according to claim 1 wherein said set of e en C t d conductingsurfaces are horizontal and substantially 5 UNITED STATES PATENTSparallel.

3. Method according to claim 1 wherein the upper fi ggfli i i conductingsurface is positively charged and the lower 6/1969 Heron et a1conducting surface is negatively charged, the paper-making fibers beingattracted by electrophoresis to the upper 10 HOWARD S. WILLIAMS PrimaryExaminer electrode and the water being attracted to the lower conductingsurface by electro-osmosis. s, 31, X

4. Method according to claim 1 including subjecting 204 300 the aqueousdispersion of paper-making fibers, prior to their entry between theconducting surfaces, to vibra- 15

